In conventional manufacturing liquid crystal display device process, lithographic process is a crucial step with high cost and high precise control. Array process with lithographic process is the most critical step particularly for liquid crystal display device for extremely large numbers of tiny thin film transistors are simultaneously formed on a huge substrate. Hence, decreased numbers of lithographic processes can promote manufacturing yield of liquid crystal display devices and reduce capitalized cost. From the array manufacturing process with six lithographic processes in former times to nowadays four lithographic processes, manufacturing cost of reticle and exposure times can be decreased when at least one reticle in lithographic process is reduced, and the necessary high precision steps can be reduced to progress product yield. Therefore, many researches and studies are focused on the reduction of reticles, and published many papers and patents.
Generally, the four lithographic processes can be applied to the manufacturing array process, because one reticle as well as one exposure process is used for two etching processes in which two patterns generated by the two etching processes are highly similar. A typical method is to use a reticle with optical correction such that photoresist layer after developed will generate different thickness, and thinner portion of this photoresist layer will be removed at followed etching steps to create masks with different patterns. Therefore, masks with different patterns by using one reticle and one exposure process can be implemented by using multi-etching process to replace original two masks by using two reticles and two lithographic processes.
One optical correction reticle is provided by Samsung. Slit structure is used in reticle design to make a portion of photoresist layer half tone at exposure in lithographic processes of thin film transistor array. Photoresist residue is formed in the half tone region due to insufficient exposure after developed. Thickness of the photoresist residue is thinner than typical photoresist layer, and then will be removed at some etching stage of following multi-etching process. Pattern of the photoresist mask is changed when the photoresist residue is removed, and two patterns of photoresist mask by using one reticle as well as one exposure step is implemented.
Another method for forming photoresist residue is to use reticle with different light transmissions. A lower transmission reticle is used at the region where photoresist residue is desired to be formed, and then the photoresist residue after developed is formed at the region of photoresist layer without sufficient exposure.
Another method again for forming photoresist residue is to use reticle with phase shift mask correction. The reticle with phase shift portion will generate destructive interference on the photoresist layer during exposure to result insufficient exposure at the desired region, and then photoresist residue is formed after developed.
However, the above mentioned lithographic processes all utilizes steps which development is after exposure and before baking step. The baking step after development will make profile of the photoresist layer deforms, and such deformed photoresist layer will make the following etching process fail.
Moreover, the reticles, no matter slit structure, varying transmission rate reticle, or phase shift mask, all are using optical correction. Nevertheless, reticles with optical correction will greatly increase manufacturing cost thereof. Using reticles with massive areas is an inevitable way especially for liquid crystal display device manufacturing from fourth generation to fifth generation, even to sixth generation, process. It is very difficult that extremely precise optical corrections are used in such a huge reticle and all optical corrections need to be identical everywhere. Hence, manufacturing yield of reticles is decreased significantly and even impractical.
Even though the above lithographic process can be implanted, other regions will be over etched at the following multi-etching steps to change patterns. A significant situation is that data line is opened at the mulit-etching steps.
Therefore, it is an important issue to provide another lithographic process for single reticle with multi-etching steps to preserve profile of developed photoresist layer and to generate a reticle with high production yield and lower cost.